Surface treatment of molds



Patented Nov. 18, 1952 UNITED STATES PA/TENT OFFICE SURFACE TREATMENT OFMOLDS Frederick 0. 'lraenkner, Massena, N. Y., assignor to AluminumCompany of America, Pittsburgh, Pa., a corporation of Pennsylvania NoDrawing. Application August 17, 1949,

Serial No. 110,883

In those casting operations wherein a porous.

mold is employed and the cast article must slide over the walls of themold cavity, the molten metal tends to enter the pores of the mold withresultant separation of a portion of the surface layer of the casting asit moves or slides through the mold. The adherence of metal to the moldresults in a tearing of the casting which generally would because forrejection even though but a small part of the total surface of the castarticle is thus affected. This is especially objectionable in continuouscasting processes where,

the ingot passes through a mold and a small area of adhering metal cancause a long broken streak on the ingot surface. It has also beenobserved that adherence of metal to the mold often occurs in certainregions of the mold surface which are related to certain combinations offreezing and heat extraction that develop during solidification of thecasting.

The material of which a mold is made generally exerts an influence uponthe surface quality of the casting formed therein. For example, somematerials may be "wetted" by molten metal while others are not soaffected. Furthermore, some of the materials are porous to the extentthat the pores tend to be filled with molten metal as mentioned above.Generally a metal mold made from plate or heavy sheet is notsufli'ciently porous to offer any difllculty on this account, however,if the mold is a'casting or a sintered product made from powderedmaterials, or if it is made from non-vitrified or compressednon-metallic substances it may have a degree of porosity which,interferes with obtaining a cast body with a ismooth surface. Inasmuchas porous molds often have certain other highly desirable qualities, itis particularly important that the dimculties associated with theirporosity should be overcome and thus permit the production of highestquality castings. One of the mold materials which is highly desirable inmany ways is graphite but molds of this material usually are porous tosome degree and therefore subject to the disadvantage stated above.

Mold washes and lubricants have been proposed and used for dressing moldsurfaces. Generally speaking, mold washes are employed because of 8Claims. (Cl. 22-192) their'heat insulating properties and are not usedwhere there is relative movement between casting and the mold during thecasting operation because of their low resistance to wear and relativelyrough surface. Liquid and semi-solid oils and greases of animal,vegetable or mineral origin have been applied to mold surfaces but suchsubstances usually introduce problems such as buming, excessive fumesand the production of a carbonaceous deposit. Solid lubricants such asgraphite either alone or associated with other materials have beensprayed or brushed on mold surfaces but such coatings require frequentrenewal because they-are only applied superficially and therefor areeasily worn or rubbed 01!.-

None of these materials or methods of application have been successfullyused for providing durable, very smooth coatings which are substantiallynon porous. 1

It is an object of this invention to provide very durable, dry, smoothsurfaces on porous molds and mold parts. A further object is to providea method of treating porous mold surfaces which is simple and effectivein rendering the mold surfaces substantially non-porous. Still anotherobject is to provide a surface on porous molds which does not spell orbecome loosened in contact with molten metal. Yet another objectcomprises the provision of a method of applying a materialto the moldsurface in such a manner as to cause it to penetrate the surface of themold .body. Another object is to provide a method of treating thesurface which willinsure that the surface is dry at the time it comes incontact with molten metal. These and other objects will become apparentfrom the following description and claims.

In speaking of mold surfaces herein, it is to be understood that thisrefers to that surface of a mold or any part thereof which comes in cqntact with molten metal. Thus, for exampfie, cores used in forminghollow portions of the castings are considered to be mold parts and theymay be treated in the same manner as the main mold I trated by moltenmetal, are dry and present an exceptionally smooth long wearing surfacewhich does not require lubrication. Because the treated mold surface hasno pores of a size which can be penetrated by molten metal it isconsidered to be substantially non-porous. The treatment, it isbelieved, introduces colloidal graphite particles into the mold poresand the subsequent baking fixes or anchors them in such a way as toprevent removal unless the mold surface itself is worn or cut away. ismost cases a much better surface is developed by repeating theimpregnating treatment before the baking operation. For convenience ofdescription, the porous mold may be said to be impregnated withcolloida1 graphite which is subsequently fixed or stabilized in such amanner as to firmly anchor its position.

The mold or mold part to be treated must possess some degree ofporosity, the pores being large enough to permit the passagetherethrough of colloidal graphite particles. It is obvious, of course,that the treatment is not adapted to the treatment of molds havingrelatively large pores for the pores must be filled with graphiteparticles to the extent of providing a smooth mold surface. It has beenfound that molds exhibiting a pore size from about 4 to 40 micronspossess a very satisfactory surface for impregnation with colloidalgraphite. While it is desirable that the pores be of about the same sizein a given mold or mold part and that the porosity'be relativelyuniformly distributed over the mold surface this is not imperative. Itis essential, however, that there be enough pores of suitable size forthe graphite particles to enter and become anchored therein.

The molds may be composed of any one of a number of materials dependingupon the properties desired. For example, sintered metallic powders, ormixtures of metallic powders and ceramic materials, or non-metallicmaterials such as graphite may be employed. Molds made of any of thesematerials may be successfully treated with colloidal graphite accordingto my invention providing they possess the proper porosity. One of thematerials which has been very successfully treated is that known in thetrade as dense graphite. This material has a porosity of from about 20%to about 30%; i. e. 20 to 30% of the volume consists of voids, and ithas a pore size of at least 4 microns. This material when in the form ofmolds and treated with colloidal graphite has been used to producealuminum alloy castings having exceptionally smooth surfaces.

The colloidal graphite preparation which I employ comprises graphiteparticles suspended in a volatile liquid or carrier. The predominantportion of particles should be of colloidal size. Because of this fact,the preparation is herein referred to as a colloidal graphitesuspension. In my preferred practice, the suspensionconsists of graphiteparticles about 1 to 8 microns in diameter. Particles of this size aregenerally considered to be of colloidal size. In general any graphiteparticles which are capable of being placed in colloidal suspension andwhich can enter the mold pores are considered to be satisfactory for thetreatment of porous mold surfaces.

The proportion of graphite to liquid must be carefully controlled inorder to secure the desired penetration of the mold. It has been foundthat the graphite should not exceed about by weight of the liquid inwhich it is suspended. A greater proportion impedes the impregnatingprocess and does not permit the establishment or a firmly anchoredcoating. Smaller amounts may be employed, however, for practicalpurposes the quantity should not be less than about 1%.

The volatile carrier or vehicle should belong to the class of liquidswhich has a boiling point within the range of from about l00.to 500 F.and have a sumciently low viscosity to permit penetration of the poresof the mold. A preferred group of liquids is that composed of a mixtureof hydrocarbons in which 10% are distilled below 347 F. and .not lessthan below 464 F. Examples of such hydrocarbon mixtures are petroleumnaphtha, and mineral spirits. Other organic liquids will also servesatisfactorily such as the lower alcohols. By organic liquids ismeant'hydrocarbons and their derivatives which are liquid at roomtemperature. Under some conditions water may be employed but specialprecautions may be necessary to insure complete evaporation.

Although the aforesaid liquids serve as suitable carriers for graphiteand in most cases possess the necessary wetting power to facilitatepenetration of the mold material, it may be desirable to add smallamounts of other substances which will serve to keep the graphiteparticles in suspension or enhance the wetting power of the suspensions.Such materials as metallic salts of long chain alkyl acids, arylsubstituted long chain alkyl acids, or aromatic acids are known to havethis effect upon carriers of the kind herein described. Some examples ofthese salts are nickel, zinc or calcium naphthenates, calcium stearate,calcium phenyl stearate, wax aluminum phenate, aluminum carboxylate. Theadded substances do not in any way interfere with the deposition of thecolloidal graphite particles nor the production of the desired qualityof mold surface.

The treatment of a mold or mold Dart consists in contacting it with asuspension of the type defined above, subsequently permitting thevolatile carrier to evaporate, and finally baking the mold. While asingle treatment effects an improvement, a better and more durablesurface is developed by repeating the treatment at least once, and hencethis is preferred. The suspension may be applied in any conventionalmanner but usually immersion or spraying will be the most convenientmethod to use. Following contact with or exposure to the suspension themold is dried, but in my preferred practice the contacting and dryingsteps are repeated at least once and often twice. By means of themultiple treatment, it has been found that the colloidal particlesbetter penetrate the pores of the mold to a depth that permitssubsequent firm anchorage in the pores. In the case of contact byimmersion it is advantageous to hold the mold -or mold part in thesuspension for a period of time, for example, 30 minutes. This periodmay extend for several hours but ordinarily such long exposure does notproduce corresponding improvement over a short exposure.

Thedrying operation may be conducted atroom temperature or at somewhathigher temperatures, but below 300 F., the object being solely that ofevaporating the carrier. The period required for drying will vary withthe liquid employed in the suspension and the period of exposure to thesuspension. It is not necessary that the drying be carried to the pointof removing all traces of the liquid, but it is essential that a majorportion be evaporated. A period up to 5 or 6 hours is usuallysufiicient.

After completion of at least one cycle of im aerapaa pregnation anddrying the article is then subjected to a baking operation at atemperature of between 500 and 800 F. for a period of at least minutes.This -may extend to several hours. While the baking operation serves todrive off any residual vehicle or carrier it also appears. tofix orstabilize the graphite particle in the pores of the mold with the resultthat a firmly adherent coating is produced. Because of the firmadherence and durability of the coating obtained in the foregoing mannerit may be said to be integrally bonded to the mold as compared to thesuperficial mold coatings of the prior art.

If upon test it is found that the mold surface is not smooth, theforegoing process may be repeated. Also, after the mold has been usedand some wear may have occurred the mold may be subjected to a singlecontact with a colloidal suspension followed by baking.

The mold surfaces resulting from this treatment are dry and may be usedwith entire safety in contact with molten metal. The mold surfaces aresmooth and free from pores that can be penetrated by molten metalascompared to the untreated mold surface. Furthermore, no additionallubrication is necessary. Such treated mold surfaces may be used forrelatively long periods of time without renewal as compared to a surfacethat has not been impregnated with colloidal graphite and baked. Adurable surface is of particular importance in continuous castingprocesses where the operation cannot be conveniently interrupted.

Although any type of metal may be cast in contact with molds treated inaccordance with my invention providing the casting conditions are suchas not to oxidize the coating or the mold, it has been found thataluminum and aluminum base aloy ingots having high surface quality canbe cast in such molds. Other non-ferrous metals such as magnesium andcopper and their alloys uid boiling between 100 and 500 F., said moldbody being at a temperature below the boiling point of said liquid andthe period of contact between said mold body and suspension being longenough to permit penetration of said mold body by said suspension, theproportion of said graphite to said liquid not exceeding 15% by weightof the liquid, drying said treated .mold body at a temperat e betweenroom temperature and 300 F. and pier after baking said dried mold bodyat 500 to 800 F. for a period of at least 15 minutes.

weight of the liquid, drying said treated mold body at a temperaturebetween room temperature and 300 F. and thereafter baking said driedmold body at 500 to 800 F. for a period of at least 15 minutes.

3. The method of providing a smooth, durable, substantially non-porousgraphite surface on a porous graphite mold body having a pore size ofbetween 4 and 40 microns, said method comprising immersing said moldbody in a suspension of graphite particles of 1 to 8 microns in diameterin a liquid boiling between 100 and 500 F.,

said mold body being at a temperature below the boiling point of saidliquid and the period of may likewise be cast in contact with thetreated molds with resulting advantage tothe surface quality of the castproduct.

The invention may be illustrated in the following example. A densegraphite mold of circular shape, open at both and bottom and having adiameter of about 14 inches was treated with a colloidal graphitesuspension containing about 10% by weight of graphite. The graphite moldhad a porosity of about 25% and a pore size between about 4 and 40microns. The mold was immersed in the colloidal graphite suspension inpetroleum naphtha for a period of 30 minutes, withdrawn and dried in airfor one hour at a temperature of 250 F. Following this it was againimmersed in the suspension and again dried. The process was repeated thethird time and following the drying operation, the mold was baked at 700F. for one hour. The mold as inserted in the continuous castingapparatus for making aluminum base alloy ingots. The ingots coming fromthis mold had an exceptionally smooth surface and, therefore, requiredno subsequent surface condition treatment to prepare them forfabricating operations. The mold treated in this manner was found toretain a satisfactory surface after casting 280 ingots having a. minimumlength of 100 inches.

Having thus described my invention, I claim:

1. The method of providing a smooth, durable,

immersion being long enough to permit penetration of said mold body bysaid suspension, theproportion of said graphite to said liquid beingless than 15% by weight of the liquid, drying said mold body at atemperature between room temperature and 300 F., repeating saidimmersing and drying steps at least once, and thereafter baking saiddried mold body at a temperature between 500 and 800 F. for a period ofat least 15 minutes. i

4. The method of providing a smooth, durable, substantially non-porousgraphite surface on a porous mold body having pores of 4 to 40 micronsin size comprising contacting said mold body with a suspension ofcolloidal graphite in a liquid boiling between and 500 F., said moldbody being at a temperature below 100 F. and the period of contact beinglong enough to permit penetration of said mold body by said suspension,the proportion of said colloidal graphite to said liquidnot exceeding15% by weight of the liquid, drying said treated mold body at atemperature between room temperature and 300 F., repeating saidcontacting and drying steps at least once and thereafter baking saiddried mold body at 500 to 800 F.- for aperiod of at least 15 minutes.

5. The method of providing a smooth, durable. substantially non-porousgraphite surface on a porous graphite mold having a pore size between 4and 40 microns and a porosity between 20 and 30%, said method comprisingcontacting said mold with a suspension of colloidal graphite in anorganic liquid boiling between 100 and 500 F., said mold being at atemperature below 100 F. and the period of contact being long enough topermit penetrationof said mold body by said suspension, the proportionof said graphite to 7 I said liquid not exceeding 15% by weight of theliquid, drying said treated mold at a temperature below 300 F.,repeating said contactingand drying steps at least once and thereafterbaking said dried mold at a temperature between 500 and 800 F. for aperiod of at least 15 minutes.

6. The method of providing a smooth, durable, substantially non-porousgraphite surface on a porous graphite mold body having a. pore size ofbetween 4 and 40 microns, said method comprising immersing said moldbody in a suspension of graphite particles of 1 to 8 microns in diameterin petroleum naphtha, said mold body being at a temperature below theboiling point of said naphtha and the period of immersion being longenough to permit penetration of saidmold body by said suspension, theproportion of said graphite to said petroleum naphtha being lessthan 15%by weight of the liquid, drying said mold body at a temperature betweenroom temperature and 300 F., repeating said immersing. and drying stepsat least once and baking said mold body at a temperature between 500 and800 F. for a period of at least 15 minutes.

7. The method of providing a smooth, durable, substantially non-porousgraphite surface on a porous graphite mold body, the pores of which arelarge enough to permit the passage therethrough of colloidal graphiteparticles, said method comprising immersing said mold body in asuspension of colloidal graphite in an organic liquid boiling between100 and 500 F., said mold body being at a temperature below 100 F. andthe period of immersion being long enough to permit penetration of saidmold body by said suspension. the proportion of said graphite to saidliquid being less than 15% by weight of the liquid,

holding said mold body in said suspension for a period of at least /2hour, drying said mold body at a temperature between room temperatureand 300 F., repeating said immersion and drying F. for a period of atleast 15 minutes.

steps at least once and thereafter baking said 8. The method ofproviding a smooth, durable, substantially non-porous graphite surfaceon a porous graphite mold body, the pores of which are large enough topermit passage therethrough of colloidal graphite particles, said methodcomprising contacting said mold body with a, suspension of colloidalgraphite in a liquid boiling between and 500 F., said mold body being ata temperature below 100 F. and maintained in contact with saidsuspension for a period of at least hour, the proportion of saidgraphite to said liquid being less than 15% by weight of the liquid,drying said mold body at a temperature between room temperature and 300F., repeating said contacting and drying steps at least once andthereafter baking said mold body at a temperature between 500 and 800 F.for a period of at least 15 minutes. V

FREDERICK O. TRAE REFERENCES CITED The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS Number Name Date 928,470 Monnot July 20, 19091,858,083 Goldsmith May 10, 1932 2,241,594 Gray May 13, 1941 2,245,747Barr June 17, 1941 2,246,463 Garratt June 17, 1941- 2,376,5l8 Spence May22, 1945 FOREIGN PATENTS v Number Country. Date 2,240 Great Britain of1856 OTHER REFERENCES Iron Age, February 6, 1947, pages 58 and 59.

1. THE METHOD OF PROVIDING A SMOOTH, DURABLE, SUBSTANTIALLY NON-POROUSGRAPHITE SURFACE ON A POROUS MOLD BODY HAVING PORES OF 4 TO 40 MICRONSIN SIZE COMPRISING CONTACTING SAID MOLD BODY WITH A SUSPENSION OFCOLLOIDAL GRAPHITE IN A LIQUID BOILING BETWEEN 100 AND 500* F., SAIDMOLD BODY BEING AT A TEMPERATURE BELOW THE BOILING POINT OF SAID LIQUIDAND THE PERIOD OF CONTACT BETWEEN SAID MOLD BODY AND SUSPENSION BEINGLONG ENOUGH TO PERMIT PENETRATION OF SAID MOLD BODY BY SAID SUSPENSION,THE PROPORTION OF SAID GRAPHITE TO SAID LIQUID NOT EXCEEDING 15% BYWEIGHT OF THE LIQUID, DRYING SAID TREATED MOLD BODY AT A TEMPERATUREBETWEEN ROOM TEMPERATURE AND 300* F. AND THEREAFTER BAKING SAID DRIEDMOLD BODY AT 500 TO 800* F. FOR A PERIOD OF AT LEAST 15 MINUTES.